Automobile bodies, home appliances such as refrigerators, dish washers, washing machines and the like may be primed, for example, by means of a so-called electro-immersion finish. This priming is customarily followed by an overall varnish build-up which may comprise several layers which essentially include a base filler and a cover varnish. The substrate to be primed is introduced into an electro-immersion varnish bath and coated in a few minutes. After removal from the bath, the substrate is rinsed, the applied varnish is baked on, and the primed surfaces are processed mechanically. Subsequently, the base filler is applied.
It is unavoidable that the immersion bath becomes contaminated with and contains fine particles. Accordingly, there is a need to filter the bath liquid. Is is also unavoidable that very fine particles settle on the primed surfaces and therefore a roughness is formed on the surfaces. Thus, these surfaces must be specially reworked. This rework is generally performed by mechanical processes such as sanding. The process cycle of the mechanical finishing is time-consuming and expensive.
To prevent settling of bath material, the treatment bath, i.e., the liquid of thc treatment bath, is continuously circulated within the bath and is recirculated by removing the liquid from the bath and conducting it through a so-called ultra-filtration loop and a simple filter loop. The state of the art regarding keeping the bath liquid clean is therefore unsatisfactory. It is a frequent occurrence, especially after interruptions of the operation, that the filters are clean and clear, but the substrates which are coated are dirty. In this connection, it must be mentioned that part of the dirt is removed from the bath by the substrates themselves. Therefore, the substrates are cleaner when there is a greater throughput.